1. Field of the Invention
The present invention relates to an evaporated fuel discharge suppressing apparatus for an internal combustion engine wherein evaporated fuel gas generated in a fuel tank is lead to an engine combustion chamber so that discharge of the evaporated fuel to the atmosphere is suppressed.
2. Description of the Prior Art
For preventing pollution of the atmosphere, evaporated fuel generated in a fuel tank of an automobile is introduced into an intake conduit of an engine to be burned in a combustion chamber of the engine together with supplied fuel. More particularly, the evaporated fuel is temporarily adsorbed by a canister, for example a charcoal canister, and then is introduced into the intake conduit of the engine by a negative pressure of the intake conduit when the engine is operated.
Japanese Patent Publication No. SHO 61-19962 discloses an evaporated fuel discharge suppressing apparatus wherein a purge control solenoid valve is installed in a purge conduit connecting a canister and a portion of an engine intake conduit downstream of an intake throttle valve and the purge amount of evaporated fuel gas is optimized by controlling the purge control solenoid valve by duty control.
However, if the purge control valve experiences problems and sticks in an open position, a large amount of evaporated fuel will flow from the canister into the intake conduit of the engine when the throttle valve is fully closed at idling or deceleration. As a result, the air-fuel ratio of the engine will vary largely to cause an engine stall or misfiring. Further, due to such engine misfiring, unburned fuel will be generated in the engine and will remain in an exhaust purification catalyst installed in an exhaust conduit of the engine. The remaining fuel may melt the catalyst when it is burned.
To solve the above-described problem, Japanese Utility Model Publication No. HEI 2-61173 proposes to install a control valve in the purge conduit in series with the purge control valve so that the control valve can close the purge conduit even if the purge control valve is stuck in the opened position.
However, even in such an evaporated fuel discharge suppressing apparatus wherein the purge control valve and another control valve are arranged in series in the purge conduit, there is a problem that if the purge control valve or the control valve experiences problems and sticks in the closed position, the evaporated fuel cannot be purged into the intake conduit of the engine so that collecting evaporated fuel will eventually exceed the adsorption capacity of the canister and will be discharged into the atmosphere via a canister vent port, resulting in a gasoline odor.